WHEN TO CHANGE OIL IN VEHICLE NOW

1981 GMC Truck • 235,000 miles

I retired my 1981 pickup & got a Honda Civic. How often should I change oil in the pickup. Drive it once a month maybe. It mostly sets. Keep battery up with Battery Tender. Drove it 410 miles in 2 years. Keep Stabile in the gas. Oil life now would be judged in time not milege. How should I handle the oil in this vehicle now?